How Do You Use An Image As The World Background?
Setting a World Environment Background Texture in Cycles

You can easily set a very nice background image for your world by creating an Environment Texture for your World. 

You can use many different kinds of background images, but one of the best types for creating a beautiful 360° background is an HDRI (High Dynamic Range Image). These types of images are designed to wrap around your world in a spherical shape so no matter where you look in the scene, you still see the background image:



There are many sources for free HDR images -- HDRI Hub is one. You can also see many examples in Google. 

Here's how to set one up in your scene in Cycles:


Make sure you are using the Cycles Render engine and make sure you are in perspective view, if not press (Numpad-5),


Set your World's Surface to an Environment Texture and use an HDR image for the background.  


Object Properties window > World tab > Surface section > click Use Nodes
For Color, click on the options button (  ), then select Environment Texture
Then click Open and select your HDRI file. Here, I'm using "Street in the woods": 



However, in order to see your background image while working on your scene, you MUST be looking through your Camera (press Numpad 0 to switch to it) and also you have to set the 3D Viewport Shading to Rendered: 



Now when you render your scene, your background image will be visible wherever you point the camera.